Amplifier circuits, such as preamplifiers, are used in numerous applications. Typically, an amplifier is used in an electronic system to increase the amplitude of a weak signal created within the electronic system. A singled ended amplifier amplifies a single ended input signal by a gain factor, such that the output signal is equal to the input signal multiplied by the gain factor. Differential amplifiers are a particular type of amplifier wherein the differential input signal comprises a positive component and a negative component. The amplifier increases the amplitude of both input signal components by a gain factor. The ability of the amplifier circuit to faithfully reproduce the input signal is a function of many factors, including inter alia, the bandwidth of the amplifier, the frequency and/or the amplitude of the input signal, the impedance of the input system, or an associated sensor, such as a disk-drive magneto-resistive (MR) head, and transmission line impedance that provides the input signal, and the input impedance of the amplifier.
In order to obtain a relatively constant gain over the specified bandwidth of an amplifier, it is desirable to match the input impedance of the amplifier and the impedance of the input system and transmission line impedance that provides the input signal to the amplifier. If a significant mismatch exists between the input impedance of the amplifier and the system providing the input signal, signal reflections at the input to the amplifier caused by the impedance mismatch will compromise the performance of the amplifier. In such a situation, the output signal will be degraded, consequently narrowing the bandwidth over which the system can effectively operate.
The impedance of the system providing the input signal can vary significantly according to the particular system and application. In a disk-drive system, the electrical resistance of an MR head changes in response to variations in magnetic flux. The MR head is moved over the surface of a disk in order to read the data stored on the disk. The MR head reads data from a disk by sensing flux changes (e.g., changes in polarity) on the magnetic surface of an associated disk as the magnetic surface passes beneath the MR head. The flux change causes a change in the resistance of the MR head. As a function of the change in resistance, the MR head provides a corresponding change in voltage. This voltage is provided to the input of a differential amplifier which amplifies the signal for use by other components in the system.